1. Field of the Invention
The present invention relates to a zoom lens system which is suitable for a lens-shutter camera and the like.
2. Description of the Prior Art
Various zoom lens systems for lens-shutter cameras have been proposed. For example, in a zoom lens system of the prior art as disclosed in Japanese Unexamined Patent Publication No. Hei-5-11181, the lens arrangement is simple; however, the zoom ratio is low. Furthermore, in the case where the number of lens elements is small, as disclosed in U.S. Pat. Nos. 5,434,711; 5,973,849, 6,236,515 and 6,067,196, a three-lens-group zoom lens system is employed, so that the zooming mechanism thereof becomes complicated, and low production cost thereof cannot be achieved. In the case of a large zoom ratio as disclosed in Japanese Unexamined Patent Publication No.Hei-11-84236, the amount of change in the overall length thereof upon zooming is too large, so that it is difficult to constitute the lens barrels therefor. Moreover, in a lens system disclosed in U.S. Pat. No. 6,154,323, the number of lens is small; however, expensive diffraction optical elements are employed therein, so that the production thereof is difficult, and higher-order diffracted light could become flare.
The present invention provides a low-cost zoom lens system having a smaller number of lens elements and the zoom ratio of approximately 3.1.
As an aspect of the present invention, there is provided a zoom lens system including a positive first lens group and a negative second lens group, in this order from the object. Zooming is performed by moving each of the positive first and negative second lens groups in an optical axis direction.
The positive first lens group includes a positive first lens element having a convex surface facing toward the object, a negative second lens element, and a positive third lens element, in this order from the object.
The negative second lens group includes a fourth lens element which is formed as a positive meniscus lens element having the concave surface facing toward the object, and a fifth lens element which is formed as a negative meniscus lens element having the concave surface facing toward the object, in this order from the object.
The zoom lens system satisfies following conditions:
xe2x80x831.2 less than ft/f1 less than 1.8xe2x80x83xe2x80x83(1)
3.8 less than ft/f1G less than 4.2xe2x80x83xe2x80x83(2)
1xe2x89xa6SF1 less than 6xe2x80x83xe2x80x83(3)
wherein
ft designates the focal length of the entire zoom lens system at the long focal length extremity;
f1 designates the focal length of the positive first lens element;
f1G designates the focal length of the positive first lens group, and
SF1 designates the shape factor of the positive first lens element, defined as SF1=(r2+r1)/(r2xe2x88x92r1),
r1 designates the radius of curvature of the first surface of the positive first lens element; and
r2 designates the radius of curvature of the second surface of the positive first lens element.
The zoom lens system preferably satisfies the following conditions:
1.50 less than n1p less than 1.68xe2x80x83xe2x80x83(4)
xe2x88x9281.2xc3x97n1p+172.7 less than xcexd1p less than xe2x88x92250xc3x97n1p+468xe2x80x83xe2x80x83(5)
wherein
n1p designates the refractive index of the positive lens elements in the positive first lens group; and
xcexd1p designates the Abbe number of the positive lens elements in the positive first lens group.
The zoom lens system can satisfy the following condition:
0.45 less than xcex94X/ft less than 0.50xe2x80x83xe2x80x83(6)
wherein
xcex94X designates the traveling distance of the positive first lens group upon zooming from the short focal length extremity to the long focal length extremity.
Furthermore, the zoom lens system satisfies the following condition:
xe2x88x923.0 less than xcex94ASP2xc3x971000/fs less than xe2x88x921.0xe2x80x83xe2x80x83(7)
wherein
xcex94ASP2 designates the amount of asphericity on the object-side surface of the negative second lens element.
fs designates the focal length of the entire zoom lens at the short focal length extremity.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2001-309656 (filed on Oct. 5, 2001) which is expressly incorporated herein in its entirety.